Here is their definition: "A Pigouvian tax [...] is a tax on a market transaction that creates a negative externality, or an additional cost, borne by individua

Here is their definition: "A Pigouvian tax [...] is a tax on a market transaction that creates a negative externality, or an additional cost, borne by individuals not directly involved in the transaction." The "additional cost" is a clarification of the "negative externality", not a condemnation of the "tax on a market transaction". I suppose it's ambiguously constructed as a sentence and we can read into it what we will.